Polymer blends of ethylene-propylene (EP) polymers or of ethylene-propylene-diene (EPDM) polymers with poly-.alpha.-monoolefin polymers, particularly with polypropylene, are known to the art (see U.S. Pat. Nos. 3,036,987; 3,262,992; and 3,835,201, and British Pat. No. 958,079 and Canadian Pat. No. 602,151). Such blends are useful to prepare a broad range of molded products such as tubing, toys, and household and automotive items. For many applications, the item must have high structural integrity and good tensile strength. Unfortunately, polymer blends of EP or EPDM polymers with polypropylene often exhibit low tensile strengths. To achieve higher tensile strengths, curing or crosslinking agents have been added to such polymer blends to effect chemical changes in the blend (see U.S. Pat. Nos. 3,564,080; 3,758,643; and 3,803,558). The polymer blends of the present invention are thermoplastic physical blends of specific EP or EPDM polymers and crystalline polypropylene (PP) polymers, which blends exhibit unexpectedly superior tensile strength. No curing or crosslinking agents are used in the polymer blends.